In a usual TCP (Transmission Control Protocol) operation, upon receiving acknowledgement (a TCP ACK packet) from a client, a server operates as if the server deemed that all the TCP packets were received up to a TCP packet with a sequence number indicated in an acknowledgement number (Acknowledgement Number) included in the acknowledgement.
Basically, a server transmits acknowledgement to a client each time a TCP packet is received from the client. Here, a technique has been known to reduce a number of acknowledgements to be transmitted from the server to the client (cf. Patent Document 1, for example).
According to Patent Document 1, for a case where multiple acknowledgements are to be transmitted to a client, a server discards an old acknowledgement, and transmits only a new acknowledgement to the client.